In trying to upgrade my car stereo, I want to replace some remote gain control pots with digital potentiometers located next to the amplifiers, and get rid of a lot of signal path cable by using digital encoders to control those pots.

I won't get into the gory details of the system layout, other than to say it is quite complex and this idea will get rid of nearly 150 feet of RCA cable, and in the process hopefully will get rid of some alternator noise at the same time.

The problem is... we have been unable to find any digital potentiometers that can handle more than about 5 volts signal measured peak to peak. I need at least 12v, and 15 would be better.

Do such a digitally controlled potentiometers exist, and if so where can I buy them?

tanstaafl.

i'd check maxims digipot section if you have a symetrical supply of +-15V i'd go with the MAX5437 or similar.

I think the DS1808 would be better as it has a +/- 12v audio taper. It's also dual channel. Although the interface appears to be i2c instead of discreet push-buttons. You can get yourself some free samples.

Hi Folks,
Though I'm new to the board, I've heard about it for years from my friend Doug B. Repeatedly, actually.

Almost a year ago, we conceived this notion to replace the hundreds of feet of signal-carrying cable in his SHO Taurus with remotely controlled volume controls. The environment and the application dictate quite a few design criteria:
- power supply is single ended 12v
- ambient temperature varies from -40C to +40C
- his car stereo is amazing, so SNR should be at least 100dB
- devices should "remember" their last setting (so poor Doug doesn't have to reset all the controls every time he turns the system on)
- the signal incident to the power amps is about 14vp-p (hot!)
- a DIP package would be nice for breadboarding

A final criteria is sort of imposed by me. These chips that require a PIC or some other microcontroller intimidate me. So, since I'm the guy that's gonna build it, I'm looking for something that is also simple and adheres to KISS. Could be that I'm looking for the Holy Grail of circuits... Time will tell.

So -- both the Maxim and Dallas "digipots" are excellent suggestions from the analog voltage perspective. However, neither one stores the wiper position in memory and power up in the "wiper at low" state. Could it be that what we look for doesn't exist?

Perhaps a VCA would do the trick? Perhaps a speedometer cable routed from the armrest back to an actual potentiometer?

Anyway, I thought that I'd post my thoughts and see what happens.

Cool BBS!

Cheers,
Tom

DS1808

MAX5437


Thanks, both of you... but I failed to list one of the critical specifications: the wiper memory has to be non-volatile. I can't be in the position of having to re-tune my system from scratch every time I turn on the ignition!

Although... hmmmm... y'know, those digital pots don't draw any significant amount of current. I suppose I could just leave them "hot" all the time. I'll have to check with my electrical engineer friend (he's the one who is designing this system; my total knowledge of electronics is that light bulbs unscrew counter-clockwise).

Tom (my engineer) had already looked at and rejected one of these chips (don't know which one, but probably the MAX5437) because in addition to not storing the wiper position, it uses a clocked, serial input requiring a microcontroller; and it is a surface mount chip which we want to avoid if at all possible.

One more parameter to add to the mix: it should have a temperature range that extends down to -40 degrees (C or F, take your pick!) and +60 C on the high end would be good too.

edit: Oh, and audio taper as opposed to linear would be nice, but not essential.

Any more ideas? Keep 'em coming if you can, I really appreciate it.

tanstaafl.

Me bein' a mechanical engineer and all, I like this thing:

http://www.allelectronics.com/cgi-bin/category.cgi?category=470&item=MPOT-10K&type=store

Me bein' a mechanical engineer and all, I like this thing:

http://www.allelectronics.com/cgi-bin/category.cgi?category=470&item=MPOT-10K&type=store



I tried them, and they do indeed do the job. I went with the 20K version, rather than the 10K.

However... they were pretty low quality from a mechanical standpoint. We damaged three of them just securing them to the faceplate of the box they are installed in. The threaded portion is so flimsy that light tightening (maybe 2--3 ft-lbs) actually collapsed the threads onto the potentiometer shaft, making three of them non-functional until I did some Dremel work and removed that portion of the pot. I suspect in so doing I damaged some of the internal workings as well, because two of them subsequently exhibited wonky behavior, refusing to attenuate all the way down.

All that being said... I just ordered 10 replacements (on the theory that at least half of them would turn out to be usable) because in quantities of 10 they were only $3.50 apiece. Why not spend a bit more and get better ones, you ask? Because really good motorized pots cost in the vicinity of $400 each! That's not a misprint: The pots I wanted to use came out to three hundred and ninety five dollars each. Plus shipping.

In all fairness, those were pretty decent pots... very high precision, with dual, sealed ball bearings in the potentiometer section and variable speed motors. And I did get a quote for some lesser pots that came in at about $140 each.

So, we'll redo the "pot box" with more of those cheap pots as replacements treating them as though they were made of spun glass and hope for the best.

Thanks very much for the link!

tanstaafl.

I would recommend epoxying them in place, carefully, rather than using the nuts if they're that fragile. A good tight fitting hole and a couple of drops of 5 minute epoxy and you should be fine. If you need to remove it, local heat and some isopropyl alcohol will get it off again.

pca

I would recommend epoxying them in place, carefully, rather than using the nuts if they're that fragile

Yep. I'd already decided on that. In addition, I plan to epoxy all of the pots to each other so they form some kind of block, with just the shafts sticking up through the cover plate.

tanstaafl.